The present invention relates to a tunable circuit component, and more particularly, to a tunable capacitive device with linearization technique employed therein for linearizing the capacitance-voltage (C-V) characteristic thereof.
Tunable capacitive devices, such as varactors, are commonly used in a variety of applications. For example, a voltage-controlled oscillator (VCO) has tunable capacitive devices implemented therein. Besides, the VCO is also a common circuit component used in a variety of applications. Taking a conventional VCO modulated transmitter architecture for example, a VCO-based modulator is used to modulate the baseband information to a radio-frequency (RF) carrier through a VCO control voltage. For instance, the information to be transmitted is applied to an input of a sigma-delta modulated synthesizer which enforces the VCO to follow the information to be transmitted.
One general implementation of the conventional VCO is to use an LC oscillator architecture which includes at least a fixed-value inductive element and at least a tunable capacitive element. Therefore, varying the capacitive value (capacitance value) of the implemented tunable capacitive element by setting the tunable control voltage will change the oscillation frequency of the VCO accordingly. In other words, the C-V characteristic of the tunable capacitive element affects the overall voltage-frequency (V-F) characteristic of the VCO. In a case where the C-V characteristic of the tunable capacitive element is nonlinear, the V-F characteristic of the VCO becomes nonlinear; however, the nonlinear V-F characteristic of the VCO causes the loop bandwidth of the aforementioned VCO modulated transmitter to change during modulation. If the loop bandwidth becomes too smaller, a signal distortion of the modulation output may occur, which degrades the modulation accuracy greatly.
Therefore, how to design a tunable capacitive element satisfying requirements of a designated application becomes an important issue to the designer.